Sound recording apparatus



Dec. 8, 1942. H. BELAR I SOUND RECORDING APPARATUS Filed Dec. 28, 1940*PpteubdDeaSJMt UNITED STATES TENT 'T SOUND RECORDING APPARATUS HerbertBelar, Philadelphia, Pa., assignor to Radio Corporation DelawareApplicationDecember 2s,

5 Claims. (Cl.

This invention relates to sound recording apparatus and moreparticularly to a ground noise reduction amplifier for "use inconjunction with such apparatus. T 5 T T In photographic sound recordingapparatus as now used commercially, the clear portion of the filmiaroundthe sound track is usually obscured byl Tan appropriate masking orenvelop irecordT which prevents the transmissionof unnecessary T lightthrough the film, while with variable density records the averageexposure is varied to accomplish the'same result. T T T the productionof such a record, an amplifieris used which amplifies and rectifies thevoice currents so as to produce a direct current of variable amplitudecorresponding to the vol- "ume: of the sound to be recorded and thisen-T velope current is used in'turn to controlthe shutters, T galvanometermaybe. T

or light valve, as'the case In such amplifiers, as hertoforeconstructed,

there has been utilized an audiofrequency amplifier which functionstobring the audio frequency input to the proper level andis followed byarectifier and a filter. .The rectified and filtered current is thenusedto control a D. C. ampllfier, the output from which actuated theground noise reduction apparatus.

The filter whichwas necessary to eliminate the audiofrequency ripplefrom the output of the rectifier necessarily produces a time delaycommensurate with the lowest frequency which it is designed to filterout and this time delay produces a corresponding retardation of theresponse of the ground noise reduction mechanism.

This necessarily produces a certain amount of clipping of the peaks ofthe waveswhen there is any rapid increase in signal amplitude.

In the present apparatus TIdecrease the adverse effects of the filtercircuit and improve the operation of the apparatus by adding a volumecompressor to the amplifier. The amplifier itself includes a tube ofthevariable gain type of America, a corporation of 1940, Serial No372,199 179-1003 which minimizes the clipping of peaks. The compressorthen comes into-operation and decreases the amplification so as toprovide only an appropriate margin during the steady state condition.Conversely, if there is a sudden decrease in the input signal strength,the compressor does not immediately change the amplification and if thesignal then returnsto its original strength no over-shooting or clippingof peaks occurs. This feature is particularly advantageous in the caseof complex waves where the fundamental may have a very low frequencywhich the ordinary groundnoise reduction amplifier would tend to followwhile it may be quite rich in over tones which would normally actuatethe usual ground noise reduction amplifier. In

T such a case the ordinary ground noise reduction amplifier \tends toreduce the apparent amplitude of the fundamental while my improvedamplifier permits the transmission of the signal substantiallyunchanged.

One object of the invention is to provide an improved ground noisereduction amplifier.

Another object of the invention is to provide an improved soundrecordingapparatus.

Another object of the invention is to provide I a ground noise reductionamplifier with compression.

Another object is to provide an improved compressor circuit for a groundnoise reduction amplifier.

Another object is to provide a ground noise reduction amplifier in whichthe time delay of i the filter circuit is substantiall compensated.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a readingof the following specification and aninspection of the accompanying drawing in which the singlefigureillustrates a commercial and the amplification is set at such a pointthat i the application of compression is necessary to preventover-amplification of the input signal. The compressor circuit itself isso designed that it has a time delay commensurate with, although notnecessarily the same as, the filter eircuit. Thefresult of thisarrangement is that when there is a sudden increase in signal it istrans mitted by the amplifier at the gain for which the amplifier issetfo r the signalTof lower amplitude thereby causing a rapid opening ofthe ground noise reduction shutter or other device type of ground noisereduction amplifier with my improved compressor circuit applied thereto.

The amplifier itself is, in general, of. commercial form and inasmuch asthe resistor and capacitor values and tube types are indicated in thedrawing no detailed description is necessary except insofar as thedesign is not commercial.

The audio frequency input is fed through the transformer ID to theamplifier H whichis of the variable gain type, and is providedwithappropriate volumecontrol devices. The output from the tube H is, inturn. fed to the amplifier l2, the output-of which is fed throughatransformer and remote control switch I5 to the rectifier 13. Thisrectifier i3 is slightly different from that usually used in that it isconstructed OFFICE as described and claimed in .Kellogg applicationSerial No. 237,603, filed October 29, 1938, Paten No. 2,227,906, issuedJan. 7, 1941.

The rectifier l3 charges the capacitors l1 and I8 simultaneously butthese capacitors have different discharging times. The capacitor l1discharges directly across the resistor I9 while the capacitor l8discharges across the resistor IS in series with the resistor l6.Therefore, the opening time of the ground noise reduction shutter isdetermined by the charging rate of the capacitor 18 while the closingtime is determined by the discharge rate of the two capacitors incombination. The capacitor 8 obviously cannot discharge through theresistor |6 until the capacitor H has reached a lower voltage than thecapacitor l8.

The output from this rectifier circuit is amplified by the D. C.amplifier tube H, the plate current of which passes to the outputcircuit indicated, whence it is conducted to the ground noise reductionshutter or equivalent device.

A portion of the output of the audio amplifier 2 is directed through thelead 20 and the resistor 2| to the potentiometers 22v and 23.Appropriate amounts of the audio frequency current are taken off of thepotentiometers 22 and 23 and applied through the capacitors 26 and 21 tothe two rectifiers in the tube 40, the rectifier circuit being completedthrough the resistors 24 and 25. This rectifier tube 4|] provides thebias for volume compression applied to the tube The cathodes of therectifier 40 are connected to the biasing potentiometers 33 and 34 whichprovidean opposing bias in a manner and for a purpose describedhereinafter, and the current passing through, the portion of thesepotentiometers above the contact passes through the resistor 35 and isreturned to the opposite end of the resistors 24 and 25. The currentpassing through the resistor 35 I develops a potential across thatresistor and across the capacitor 36 which potential is applied throughthe resistor 31 to the control grid 38 of the tube where it serves todecrease the output of the tube on an increase in signal strength. p

The current from the rectifier 40 also charges the capacitor 4| throughthe resistor 31 and through the upper portions of the potentiometers 33and 34. The timing of the compressor circuit is therefore determinedprimarily by the values of the capacitors 36 and 4|, the resistors 31and 35, the upper portions of the potentiometers 33 and,34 and theresistors 24 and 25 in the order'named. The upper portions of thepotentiometers 33 and 34 and the resistor 3! affect principally thecharging time of the capacitors 36 and 4|, while the resistor 35 affectsprincipally the discharge time thereof,

A positive potential is derived from the plate supply of the apparatusthroughv the resistor 3|), the resistor 3|, and the resistor 32, andthis positive potential is applied across the potentiometers 33 and 34.These potentiometers may i be so adjusted as to develop any appropriatebucking potential opposing the passage of current through the twoelements of the rectifier 40. No current will pass and, therefore, therewill be no volume compression when the poten-- tial across the rectifier40 is not greater than the minimum potential applied byeither thepotentiometers 33 and 34. As the potential applied to one of therectifying elements reaches the miniratus increases, the current throughthis portion. 3

of the rectifier will increase and at a predetermined point determinedby the other potentiometer the other half of the rectifier will alsostart to pass current, thereby increasing the rate of increase ofvoltage drop across the resistor 35 with increase of inputv anddetermining the.

shape of the compression curve. Since the two potentiometers 33 and 34are identical, either may be used to determine the minimum input atwhich compression will start and the other may be'used to determine theshape of the compression curve.

In the normal operation of the amplifier the switch 50 remains in itsupper; position, as shown in the drawing. If it is desired to test'theamplifier or to operate it for any other reason without having itconnected to the normal output circuit, the switch 50 should be moved tothe lower position thereby connecting the 760 ohm resistor 5| across theoutput. The resistor 5| has substantially the same resistance as theusual output load and therefore if this resistor is substituted for theoutput circuit the amplifier may be operated in its customary manner forthe purpose of adjustment.

It will be apparent that there will be a certain amount of time delaynot only in the input circuit to the rectifier 40 but also in the inputthrough the potentiometers 33 and 34 to the capacitor36 and the variousdelays in this compressor circuit may be 50 adjusted as to produce themost desirable timing efiect in conjunction with the filter circuit inthe output of the rectifier l3. The delay in the compressor circuit maybe'either greater or less than that in the filter circuit, and dueconsideration should also be given to the delays which occur between theamplifier tube and the rectifier in the audioi'requency circuit. Insofaras I am now aware, the values of the several resistors and capacitorsshould be substantially as indicated in the drawtion of the ground noisereduction apparatus just prior to the efiect of the compression circuiton the first input tube.

Having now described my invention, I claim:

1. An amplifier including an A. C, amplifier, a rectifier, and a filtercircuit, and a compressor circuit connected between the output and inputof said A. C. amplifier, said compressor circuit having a time constantof the same order of magnitude as the said filter circuit.

2. A ground noise reduction amplifier including an audio frequency inputcircuit A. C. amplifier, a rectifier, a filter circuit, a D. C.amplifier and an output circuit from said -D. C. amplifier forconnection to ground noise reduction appar'atus, and a compressorcircuit connected between the output and input of said A. C. amplifier,said compressor circuit having a time constant of the same order ofmagnitude as the said filter circuit,

3. An A. C. to DC. amplifier including an A. C. amplifier, a rectifier,a filter circuit, a D. C. amplifier, and a compressor circuit connectedbetween the output and the input of said A. C. amplifiensaid compressorcircuit having a time constant of the same order of magnitude as thesaid filter circuit.

4. In combination with a signal input circuit,

a filter provided with input and output circuits and a rectifierconnected between said input circui ts, a compressor circuit connectedto said signal input circuit and having a. time constant 01 5 n the sameorder of magnitude as that of said filter and means for adjusting. boththe dynamic characteristics and the time constant of said amplifier.

5. An amplifier including an A; C. amplifier, a rectifier, and a filtercircuit, and a compressor circuit connected betweenthe output and inputof said A. C. amplifier, said compressor circuit including a rectifiercircuit having a time constant or the same order of masnitudeas the saidfilter circuit.

HERBERT BELAR.

